1. Field of the Invention
The present invention relates to fiber lasers and more particularly to a method and device for pumping a fiber laser.
2. Information Disclosure Statement
The radiation emitted by a laser beam source can be coupled into an optical fiber of suitable dimensions and optical properties wherein light can be transported with no significant loss over very long distances. Today's state of the art optical fibers have found broad application in the fields of telecommunication, optical inspection, medical therapy, laser applications and many more.
Since it is possible to couple into optical fibers the radiation of high power light sources, such as diodes and laser beams, one can envision producing specially manufactured illuminated fibers for a wide variety of applications. Gallery lighting for sensitive artwork is an excellent candidate for a system with reduced risk of electrical fires. Emergency illumination systems for aircraft, trains, boats, swimming pools, and other areas where standard electric wire-based illumination would be undesirable is another application. Such applications require a significant illumination source. The problem of obtaining a sufficient illumination source for such demanding applications can be solved in an elegant way by the application of a fiber laser.
A fiber laser generally consists of a fiber core that is doped with a laser active material, an undoped inner cladding, which is sometimes referred to as the pumping ‘core’, and an outer cladding to maintain the wave-guiding effects of the inner cladding. FIG. 1 illustrates such a conventional fiber laser, which is end pumped into the inner cladding. It consists of fiber core 102, inner cladding 104 and outer cladding 106. The laser active material, also known as a gain medium, within fiber core 102 is typically a rare earth element, such as Erbium, Neodymium, Ytterbium, Holmium, Europium, and Samarium. A radiation source pumps radiation into doped core 102 through outer cladding 106. The laser active material in core 102 absorbs the pump radiation and reemits light in another wavelength from an electronic transition. A portion of this light is emitted in the direction of fiber core 102 and guided therein. Multimode inner cladding 104 acts as a pump cavity around laser core 102 to couple radiation to the gain medium. The radiation entering multimode inner cladding 104 totally internally reflects the pump radiation so that it passes through core 102 many times to effectively pump the laser. Outer cladding 106 prevents light from propagating out of inner cladding 104. Outer cladding 106 is usually made of polymer material or low index glass.
If the two fiber ends are coated in a suitable manner, for example one with a predefined degree of reflection, so a part of the light can couple out to form the laser beam and the other one coated to be highly reflective for the laser wavelength, laser action is possible in fiber core 102. Bragg gratings may also be used at the two ends of a fiber to accomplish the required effect.
An important consideration is to have the energy of the pumping light source concentrated at the absorption bands of the dopant materials being used. In order to produce an inversion of population in the laser material, the light source must have sufficient energy at the absorption bands of the dopant material.
Preferably, fiber core 102 and its claddings are designed so that only a single mode can propagate therein. Inner cladding 104 surrounding fiber core 102 has a lower index of refraction than core 102. Outer cladding 106 has an even lower index of refraction and forms an outer fiber itself surrounding inner cladding 104. Outer cladding 106 may also serve as the protective coating for the fiber. Radiation 108 is coupled into this so-called outer fiber, exciting the dopant in inner core 102, thus pumping the laser.
The pump-radiation is coupled into inner cladding 104 and is guided therein. It causes the necessary inversion over the fiber length. Fiber core 102 is designed in order to propagate the single mode only. The pumping process drives this mode, so if the endfaces of the fiber are polished and suitably coated, a laser action operation starts. Due to the homogeneous pumping method the intensity of the laser mode is equal along the fiber. Generally, the radiation of today's fiber lasers has a wavelength around 1.3–1.5 microns.
To utilize fiber lasers for illuminated fibers, at least two changes have to be applied to the state of the art concepts. The radiation must be made visible and the pumping mechanism must be improved to provide greater quantities of light.
If the fiber core is pumped directly from the side or its end, a second cladding may not be necessary. This would especially be true where the doped core is a multimode core. Multimode cores also have greater illuminating capacity than single mode cores, and are preferred in the applications considered for the present invention. Since multimode fibers must be pumped harder than a single mode fiber, high power laser diode sources are frequently used as the pump source. The use of laser diodes as a pump light source is advantageous due to the relatively high power of the laser light. A concern when using laser diodes as a pump source is that laser diodes do not emit rotationally symmetrical radiation. The angle of dispersion in the region parallel to the mounting surface is smaller than that of the perpendicular region. The light beam does not diverge as quickly in the direction parallel to the mounting surface compared to the divergence of the beam in the direction perpendicular to the surface. If an optical system, such as a lens, is not used to focus the light energy, part of the energy will not be coupled into the fiber and will be lost.
U.S. Pat. No. 4,815,079 describes an apparatus for coupling radiation into a single-mode core of an optical fiber, having a multimode cladding and an outer layer to prevent radiation from propagating out of the cladding. The single-mode core is disposed at an offset from the geometric center of the multimode cladding. In a side pumped preferred embodiment, a portion of the cladding is exposed and a prism or tapered wedge is affixed to the exposed portion of the cladding. Radiation, which impinges upon the face of the wedge, is coupled into the cladding. The use of multiple wedges or prisms is also disclosed. The patent restricts itself to substantially single-mode cores. There are no means described for a compact configuration or pump source to provide radiation to multiple coupling sites simultaneously.
U.S. Pat. No. 4,829,529 describes a fiber laser pumped by a high power laser diode source using a multimode fiber cavity around a single mode laser core. Pump radiation is input into the multimode fiber that totally internally reflects the pump radiation through the single mode core to effectively pump the laser. The multimode mode fiber has a much larger diameter than the single mode core and is a better target for end pumping. The invention is limited to end pumping with a single pump source to power a single mode core.
U.S. Pat. No. 5,854,865 describes a technique for coupling pump light into a fiber by injecting light through the side of a fiber. The technique relies on the fabrication of at least one groove or micro prism into the side of the fiber. A lighting means is placed on the opposite side of the fiber in close proximity to the fiber wall. Light propagates laterally through the fiber and impinges on the sides of the groove where it is reflected and directed along the horizontal axis. The use of multiple grooves along the length of the fiber is discussed. In the embodiments described, the groove extends into both the outer and inner claddings. Grooves extending into the inner cladding weaken the fiber, degrade reliability, and limit the ability of the fiber to be wound into a compact coil. If the fiber were placed in a coil configuration, light sources would have to be positioned in the center of the coil. This would be an awkward and expensive assembly.
U.S. Pat. No. 5,923,694 describes a pumped fiber laser including turns of optical fiber defining a wound pack. A fiber core with cladding is surrounded by a porous glass matrix material. A wedge device connected to the fiber directs light from a pumping laser into the side of the optical fiber at a plurality of different turns of the wound pack. The wedge device only directs light onto the surface of the cladding and does not focus or concentrate the laser light into the individual windings.
U.S. Pat. No. 6,229,939 describes a high power diode-pumped single-mode fiber ribbon laser having a rectangular shaped core. The ribbon contains a plurality of parallel wave-guides and mode filters to propagate a desired single-mode. Each of the cores in the ribbon is optically pumped from the side by a bar of diode arrays. In a preferred embodiment, the ribbon is wrapped around a mandrel. The described invention is limited to single mode fibers having a terminal output, and would not be practical for a multi-mode fiber having illuminating properties.
U.S. Pat. No. 6,243,515 describes an apparatus for optically pumping an optical fiber from the side. The apparatus comprises a grating formed on the longitudinal surface of a multimode fiber. A beam of pump light is passed through the fiber at a region where the outer cladding has been removed. The grating on the opposite surface causes the light to be diffracted at an angle that matches the propagation angle. Pump light passing through a doped central core causes the media to be optically excited. Removing the entire outer cladding from a region of the optical fiber can significantly weaken the fiber and decrease reliability. Pumping the fiber from the side opposite the grating makes it difficult to use the technique on anything but a relatively linear fiber. Compact configurations, such as coils, are prohibited or at least difficult to arrange.
To effectively use optical fibers as illumination sources a compact fiber laser pumping device capable of transferring high-energy radiation is desirable. There still exists a need for a fiber laser-pumping device that can couple light in an efficient manner and that is economical to manufacture.